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Author Topic: Yield Limits, Distances and Spacing for Wood Structures with Metal Fasteners?  (Read 1413 times)

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Offline kreyszig

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Hi,

So I have this chicken coop project I described in another thread for which I would like to use some mortise and tenon joints with lumber. I just went through the "Standard for Design of Timber Frame Structures and Commentary" from TFEC and it was a great read, but it shows that I will not be able to use wood pegs with the 4x4 posts due to the distance and spacing constraints (Table C3A). The document refers to the ANSI/AF&PA NDS 2005 Section 11.5 for joints with metal fasteners. Are these parameters available anywhere for free? I have found the Wood Handbook 2010 Edition from USDA. Is it a good reference? I want to make sure to calculate yield limits correctly and allow for proper metal fastener distances and spacing in my design.

Any other suggestion for a reference to help me design the joinery with lumber and metal fasteners?

Thanks!

Offline Don P

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no, do not use the wood handbook for design, those are the "raw" numbers rather than the allowables. that data fed into the engineers and smart people that created the supplement to the nds, free download, that is the tables of allowable design values. from those and the nds they created the span tables in the building codes. using that is like trying to make a tomato sammich the Martha Stewart way, starting with making dirt :).

the NDS does get into what you want sort of but is more of a establishing the rules document. I'll look, I did have a link to a read only version awhile back. now were starting with planting a seed.

strongtie.com has the proprietary connectors, tables of design values etc you are probably going to use, Simpson. just pick the 'mater!

awc.org has a spancalc that will work for some members, use it whenever applicable

for the holes in that I wrote some calcs in the toolbox, lower left of the page here, scroll down to my calcs> construction calcs and start banging on them, holler if you get stuck. surprisingly unplowed parts of the garden.

Offline kreyszig

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no, do not use the wood handbook for design, those are the "raw" numbers rather than the allowables. that data fed into the engineers and smart people that created the supplement to the nds, free download, that is the tables of allowable design values. from those and the nds they created the span tables in the building codes. using that is like trying to make a tomato sammich the Martha Stewart way, starting with making dirt :).

the NDS does get into what you want sort of but is more of a establishing the rules document. I'll look, I did have a link to a read only version awhile back. now were starting with planting a seed.

strongtie.com has the proprietary connectors, tables of design values etc you are probably going to use, Simpson. just pick the 'mater!

awc.org has a spancalc that will work for some members, use it whenever applicable

for the holes in that I wrote some calcs in the toolbox, lower left of the page here, scroll down to my calcs> construction calcs and start banging on them, holler if you get stuck. surprisingly unplowed parts of the garden.
Thanks! These links are very useful The connection calc from awc.org (Connection Calculator) basically gives the connection capacities I was looking for.

I guess what I am missing now are the required detailing dimensions (edge distance, end distance and spacing) such that the joints do not become limited by the double shear (horizontal and/or vertical) of the tenons and mortised members in resistance to the metal fasteners. They refer to it as the "NDS provisions for edge distance, end distance and spacing" in the TFEC standard. Do you know where I can find that? Is it what you refer to in the toolbox? I could not find it if it is the case :S

Offline Don P

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here it is, the web read only NDS;
https://www.awc.org/pdf/codes-standards/publications/nds/AWC-NDS2015-ViewOnly-1603.pdf

scroll down the contents to dowel type fasteners and start reading, lots of good info there. read the footnotes under the tables.
under "special conditions" catch 15.3, built up columns.


Offline kreyszig

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here it is, the web read only NDS;
https://www.awc.org/pdf/codes-standards/publications/nds/AWC-NDS2015-ViewOnly-1603.pdf

scroll down the contents to dowel type fasteners and start reading, lots of good info there. read the footnotes under the tables.
under "special conditions" catch 15.3, built up columns.
Thanks! It should be very helpful!

Offline kreyszig

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So I went through Chapter 12 of NDS. Regarding the minimum spacings, are the minimum chosen such that the limiting factors for the connection become the yield limits of Section 12.3 as opposed to, for example, horizontal double sheer of a principal member between a bolt and the end of the member when loaded in tension parallel to the grain?

I have read "A Timber Framer's Workshop" from Steve Chappel and he does compute in his book horizontal and vertical sheer constraints for the members at the connections in addition to the bearing and bolt bending limits which are the only modes of failure addressed directly in the NDS. Given that the minimum spacings (including the geometry factors) constrain distances and can reduce yield limits (in the case of the geometry factors), this is why I am assuming that these are meant to ensure that the Im, Is, II, IIIm, IIIs and IV yield modes become the limiting factors in a connection rather than shearing. Can anyone confirm this?

Offline Don P

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That is my understanding, if the connection geometry is correct then the weakest mode controls
more reading on this also the fastener chapter in the wood handbook;
https://www.awc.org/codes-standards/publications/tr12

http://seain.org/images/meeting/030217/2017_ISEA_Spring_Conference/awc___wood_connection_design_des330__1_slide_per_page_.pdf

http://www.structuremag.org/?p=10652

Offline kreyszig

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That is my understanding, if the connection geometry is correct then the weakest mode controls
more reading on this also the fastener chapter in the wood handbook;
https://www.awc.org/codes-standards/publications/tr12

http://seain.org/images/meeting/030217/2017_ISEA_Spring_Conference/awc___wood_connection_design_des330__1_slide_per_page_.pdf

http://www.structuremag.org/?p=10652
Thanks! These are all great!
So now I have everything to calculate tension yield from fasteners.
Do you have references for calculation of the strength of a mortise and tenon joint in compression, or in tension when strength is not only provided by fasteners? Also, for joints such as knee braces, there can be surfaces with multiple orientations with respect to the grain that are in compression. So how should I combine the strength from the fasteners and from the different surfaces in compression to obtain the total strength of the joint?
In Chappel's book (example of a shouldered/half dovetail joint in tension, page 97-102 of the latest edition), he seems to simply add them up for a given member. Isn't it a bit simplistic and aggressive though? It assumes that each loaded component (e.g. fasteners, dovetail shear area) reaches its own limit for the same applied force on the member, which does not seem to make sense to me? Shouldn't the calculation aim at computing the total applied force on the member when the limit of the weakest component is reached?
Thanks!

Offline Don P

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Yes. For instance if you were to mix say nail and bolt fastener strengths with the assumption that a combination of nails and bolts would "help" one another. Bolts take load right now, nails slip a bit before taking full load. In that scenario one group of fasteners would overload and then the next, So think through what you are doing, things don't always "hook up" the way we assume.

Compression checks are bearing area and allowable compression from the design data in the NDS Supplement. you're also getting into checking horizontal shear in joint design, single shear of a section or double in something like a peg relish.

Offline kreyszig

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Yes. For instance if you were to mix say nail and bolt fastener strengths with the assumption that a combination of nails and bolts would "help" one another. Bolts take load right now, nails slip a bit before taking full load. In that scenario one group of fasteners would overload and then the next, So think through what you are doing, things don't always "hook up" the way we assume.

Compression checks are bearing area and allowable compression from the design data in the NDS Supplement. you're also getting into checking horizontal shear in joint design, single shear of a section or double in something like a peg relish.
Thanks, I will go through the NDS Supplement.
So in the case of a knee brace, how would you combine the strengths of the shoulder and bearing end? Also, would you simply ignore the yield from the fasteners when the brace is in compression, or is there a safe way to consider it as well? Thanks!

Offline Don P

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What I think you need now is the TF Guild's Red Book. "Timber Frame Joinery and Design Workbook". Page 44 gets into this. You would break the brace axial compression load down into its horizontal and vertical components. Then using the Hankinson formula for angled bearing strength adjustment, check the vertical and horizontal bearing surfaces respective to their loads. Peg free. It then goes into the bending force that is pushing on the loaded post there. And explanation of more joint configurations.

The top report here on frame testing, but there's more good reports here;
https://www.tfguild.org/timber-frame-engineering-council/research-reports

Offline kreyszig

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What I think you need now is the TF Guild's Red Book. "Timber Frame Joinery and Design Workbook". Page 44 gets into this. You would break the brace axial compression load down into its horizontal and vertical components. Then using the Hankinson formula for angled bearing strength adjustment, check the vertical and horizontal bearing surfaces respective to their loads. Peg free. It then goes into the bending force that is pushing on the loaded post there. And explanation of more joint configurations.

The top report here on frame testing, but there's more good reports here;
https://www.tfguild.org/timber-frame-engineering-council/research-reports
Thanks! The Red Book seems nice. I wish I could buy an electronic copy (shipping to Canada from the TF Guild is $40 :S).
Following your very helpful explanation, and based on compression design values of 900 psi (parallel) and 625 psi (perpendicular) for stud grade Doug fir (from the Supplement), I get a value of 738 psi using Hankinson's. For a 2x4 knee brace at 45 degrees with a tenon length of 1.625", a tenon width of 1" and a shoulder height of 3.325", I end up with surfaces of 1.625 in2 for the bearing end and 1.662 in2 for the shoulder. Because both surfaces are at 45 degrees with respect to the grain (although at 90 degrees from each other), I use the same compression design value of 738 psi and the product with the areas give me 1198 lbs for the bearing end (that provide the vertical component of the load) and 1226 lbs for the shoulder (the horizontal component of the load). Assuming that both components should be equal given a load at 45 degree, I guess I should use 1198 lbs for both components, so the total load capacity at 45 degrees should be 1198*sqrt(2)= 1694 lbs.
That is the load within the brace though. If the brace is located at the upper third of the post, the load in the brace should be 3*sqrt(2) larger than a force applied horizontally at the top of the post, so that means this brace would be good to resist a horizontal load of 1694 / (3*sqrt(2)) ~= 400 lbs at the top of the post.
Does that make sense?
I have not considered the fasteners and bending forces within the post and the brace though. Is the NDS (Sections 3.4, 3.5 and 3.6) a useful reference for the bending force calculations? There is an obvious bending force applied on the post by the knee brace, but aren't the posts and the braces subjected to bending simply from the load parallel to grain as well? It looks like Section 3.9.2 and 15.4 of the NDS might be useful for this purpose?

Offline Don P

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dude... you followed that and worked it, that's awesome, you explain it to the next person :)
With a 45* brace compressions are pretty equal but remember the concepts, you'll see that again for example in roof work at varying angles. Be conservative.

Yup you got it, Read the section in the NDS on combined bending and axial compression 3.9.2, review 3.6.3 as far as poking a hole at the point of max moment. Also read 15.4 wood columns with side loads and eccentricity.

Thought experiment. Put a popsicle stick between thumb and forefinger and squeeze in compression. It will buckle at some force. Then squeeze down but also push in from the side of the stick at the same time. It should have buckled at a much lower axial force in the stick. That's combined bending and axial compression.

Look at the equation in 3.9.2, you are solving both bending and buckling , comparing their magnitude then making sure that combination comes up at 1 member or less needed to resist the combined forces. If you come up at greater than 1 you don't have enough meat yet. There is an older simpler form of that equation in the oldest, I think '44 version of the NDS on the publications page. (I've managed to make that older equation work online but never got the newer longer one to click.) This calc uses that older simpler equation, it is for a uniform side load rather than the braced post's concentrated load but maybe gives some more insight; You can play with the variables and get some sense of where the sweet spot is.
http://www.timbertoolbox.com/Calcs/44axbend.htm

Those calcs are all open source. I didn't get the current 3.9.2 equation to work so never tried 15.4... feel free to modify and give it a go.


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